The field of software development has undergone substantial evolution in the 50 years since its inception. An early form of software simply abstracted and represented hardware functions in a “human readable” form known as assembly language code. Special purpose application programs could then be written in such code to command the computer to perform an organized set of operations.
Assembly language coding has several drawbacks. One is that the code required to carry out a particular function has to be specifically included in each program needing that function: dozens of software developers might write code (or at least copy code) for a single function dozens of times for dozens of programs. A second drawback to assembly language coding is that programmers must understand the computer's internal hardware resources: as hardware is changed, so must the programmers' software programs.
To address these and other difficulties, the software necessary to operate a computer was soon split into two classes: operating system software, and applications software.
Operating system software (sometimes termed simply “the operating system”) is a set of computer code that performs a variety of functions, including interacting with the computer hardware; insulating the programmer from the specifics of the computer hardware; and providing general functionality (“resources”) that many application programs require.
Applications software (sometimes terms “applications”), in contrast, addresses a particular problem to be solved. Examples of applications software are word processors and accounting programs. Applications software and operating systems communicate using a series of standardized instruction codes passed between the two. The operating system software interprets these standardized instruction codes, and controls the computer's hardware resources in response thereto. Oftentimes, information in the form of data or messages is passed back to the applications software. In this manner, the operating system software—once written—effects most direct control of the hardware, allowing the application software programmer to focus on the particular problem sought to be solved. (Alternatively, application software can still include assembly language code—directly controlling the computer hardware and bypassing the operating system. This may be done for speed or control, or simply to provide functionality not supported by the operating system.)
The relationship between operating system software, applications software and hardware is not always clearly defined. Over the last several decades, each has evolved rapidly. As was noted above, operating systems sometimes evolve to include functions common to several applications. Similarly, operating systems are sometimes modified to include functionality previously provided only in hardware and visa-versa. For example, data compression can be done in software (by an operating system or application) and in hardware (by a specialized chip). Essentially, the three technologies have developed together in response to user demand and advances in technology.
A specific factor driving the evolution of operating system software has been the desire to simplify and speed application software development. Application software development can be a daunting task, sometimes requiring hundreds of man-years of effort for a sophisticated program. For a popular operating system such as Microsoft® Windows™, applications software developers may write thousands of different applications each year to execute on the Windows operating system. The investment in application development time is large. One incentive to create such programs is to provide good support for the application software through the operating system. This requires building a coherent and usable base for applications developers to work with.
Often, development of application software can be made simpler by making the operating system more complex. That is (as noted above), if a function may be useful to several different application programs, it is better to write it once for inclusion in the operating system, than requiring dozens of software developers to write it dozens of times for inclusion in dozens of different applications. In this manner, if the operating system supports a wide range of common functionality required by a number of applications, significant savings in applications software development costs and time can be achieved.
Another factor driving the continuing evolution of operating system software is the ongoing development of faster and more sophisticated computer hardware—both in the computer's central processing unit (CPU) and in its peripheral devices (e.g. printers). New CPUs typically offer enhanced instruction sets that enable certain new operations to be performed, or enable old operations to be performed more quickly. Corresponding enhancements are usually made to the operating system to afford applications access to these new CPU capabilities.
The relationship between operating systems and hardware has also driven the development of a special class of software: device drivers. Peripheral devices usually do not communicate directly with the operating system. Instead, such devices usually communicate with device driver software, which in turn communicate with the operating system. A device driver thus serves as a software interface between operating system software and peripheral device hardware. The specific requirements of a particular hardware device are often handled by the device driver, making the details transparent to the operating system. A computer system typically employs many different device drivers, e.g. one for the display, one for the printer, one for the mouse, etc.
Often, a new peripheral device can be supported just by writing a new device driver tailored to exploit its hardware; no revision to the operating system is required as long as the operating system recognizes and can communicate with the device driver. Sometimes, however, significant gains in functionality and efficiency can be achieved by upgrading an operating system's facilities for interacting with device drivers (and thus with the peripheral hardware).
It is clear that for a useful and accomplished operating system, the interfaces between the operating system and the computer hardware and application software are as important as efficient internal operation of the operating system itself.